The idea is that limewire will randomly pick one Gwebcache of the 300 at startup. the idea is to share the load of requests to all web caches. Each gwebcache store 20 differents ultrapeers so the users won't try to connect to the same UPs. Also, each cache has 10 other caches in memory so when an another one is added, your client is informed.

To answer your question, no it is not possible unless you modify the source, and if you want to connect to your cache, well startup limewire +/- 300 times! Of course your cache is benefical for all gnutella users as well as you.